1. Field of the Invention
The present invention relates to a cellular mobile telephone exchange system comprising one or more mobile stations, a plurality of groups of radio base-station transmitter-receivers, base-station controllers assigned to each group of radio base-station transmitter-receivers for controlling each radio base-station transmitter-receiver in said group, and a mobile telephone switching unit for controlling a plurality of base-station controllers.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a cellular mobile telephone exchange system of the conventional type. FIG. 2 is a diagram showing the coverage areas of each radio base-station transmitter-receiver 1a through 1f. Each transmitter-receiver 1a through if has a transmitting-receiving function for a plurality of channels and covers one area as shown in FIG. 2. Base-station controller 2a comprises communication channels communicating with a mobile telephone exchange unit 3 and radio base-station transmitter-receivers 1a, 1b and 1c, a network switch which connects these channels to each other, and a CPU for controlling the network switch. Similarly, base-station controller 2b comprises communication channels communicating with mobile telephone exchange unit 3 and radio base-station transmitter-receivers 1d, 1e and 1f, a network switch which connects these channels to each other, and a CPU for controlling the network switch.
When mobile station 5 moves out of the range of radio base-station transmitter-receiver 1b and into the coverage area of radio base-station transmitter-receiver 1c, base-station controller 2a switches the communication channel from the radio base-station transmitter-receiver 1b to 1c by using its network switch and CPU to continue the call with mobile station 5. Next, when mobile station 5 moves out of the coverage area of radio base-station transmitter-receiver 1c and into the coverage area of radio base-station transmitter-receiver 1d, base-station controller 2a becomes unable to keep up with the movement of the mobile station by a switching operation within the controller, and consequently, base-station controllers 2a, 2b and mobile telephone exchange unit 3 exchange information to connect the appropriate call channel by using the network switch of the mobile telephone exchange unit 3, thereby continuing the call with mobile station 5.
The conventional cellular mobile telephone exchange system described above has the drawback that when a mobile station frequenting moves between the coverage area of a radio base-station controlled by a base-station controller and the coverage area of another radio base-station controlled by another base-station controller according to a schedule set for more than one time zone, or when traffic increases between the coverage areas of more than one base-station controller (movement characteristics of a mobile station), the load on the mobile telephone exchange unit will increase to an excessive degree.